Decoction mashing is one of the three mashing methods that are often widely used in the production of bottom-fermentation beers, the other two methods being single infusion mashing and step infusion mashing. The process typically requires three vessels: a mash tun for mash mixing, a mash kettle (or copper or mash copper) for boiling, and a lauter tun (or clarifying tun) for straining. Mashing is carried out in a mash tun, and starts at a low temperature while portions of the mash are taken out and boiled in the mash kettle and later returned to the mash tun, thus gradually raising the temperature of the entire mash. The process is usually repeated two or three times, taking two to six hours. The mash temperature may start as low as 35° C., but more often at 45-50° C. to reach 70-78° C. at the end of the mashing process. The mash is then filtered in a separate vessel known as a lauter tun or in specific cases in the mash tun itself over a perforated bottom.
The fact that part of the mash is boiled is the main difference between decoction mashing and the other mashing methods. Because of the boiling, cell walls of the starch containing grains are destroyed. This allows an easier access for the enzymes to the starch. As a result the efficiency of decoction mashes is generally higher than for other mashing methods. Another advantage of decoction mashing is that grains that need gelatinising at high temperatures can be boiled separately in one of the decoction steps. This can be useful if an adjunct such as maize, rice or rye is employed in the mash.
Decoction mashing methods employed within the brewing industry are carried out in a batch-wise fashion.
DE-A 1 442 292 describes a batch decoction process comprising the following steps:    i. mixing malt with water and other ingredients to obtain an aqueous malt enzyme suspension;    ii. mixing malt, maize grits and water to obtain a decoction suspension;    iii. liquefying the decoction suspension by first heating at 70° C. for 30 minutes and subsequently heating at 95-100° C. for 10 minutes;    iv. combining the heated decoction suspension and the malt enzyme suspension to obtain a mash;    v. maintaining the mash at 70° C. for 45 minutes; and    vi. removing spent grain.In the paragraph at the bottom of page 4 it is stated: “In order to avoid high viscosity in the cooker, about 10% of the total malt may be added to the maize grits”.
It would be advantageous to carry out decoction mashing in a continuous fashion. Continuous operation of decoction mashing would offer a number of significant advantages, including:                higher productivity and lower investment: vessels can be operated for prolonged periods of time under full load, meaning that for equal production volume smaller vessels are needed than in a batch process;        constant and better quality: process is easier to control due to possibility of adapting process parameters to local and instantaneous requirements and because steady-state-conditions are much more stable;        high hygienic standard: continuous process is operated in a closed system.        less energy: energy consumption is evenly spread, without major use peaks;        less labour: operation of continuous process requires less attention        less standstill and cleaning: continuous process can be operated at much longer runlengths than batch processes.        
U.S. Pat. No. 3,171,746 describes a continuous method of producing wort using a double decoction process in which a mash is prepared in a mixer-separator from malt and water and separated in a thick mash and a thin mash. The thick mash is subjected to temperature conditions such that proteolysis and saccharification will occur, whereas the thin mash, which is rich in enzymes, is decocted and then reunited with the thick mash after the thick mash has completed the proteolytic and saccharification actions.
DE-A 18 14 377 describes a double decoction process for the continuous production of wort wherein a mash is prepared by combining malt and water and wherein part of the mash is fed to a vessel in which it is subjected to heat treatment before it is recombined with the remainder of the mash that has not been subjected to such heat treatment. Next, again a part of the mash is fed to a vessel in which it is heat processed before being combined with the remainder of the mash that has not been heat treated, following which the mash is separated in wort and spent grain.
WO 92/12231 describes a process for the continuous preparation of wort comprising continuous enzymatic conversion of malt in at least one rotating disc contactor. In the example of this patent application a decoction suspension containing maize and malt is maintained at 50° C. for 5 minutes, heated to 95° C. for 10-15 minutes in a rotating disc contactor, combined with malt/water mixture, following which the resulting mixture is first heated to 65° C. for 30 minutes and then to 76° C. for 5 minutes.
In this process gelatinisation and enzymatic degradation of the starch contained in the decoction suspension are achieved in a single heat treatment (i.e. 95° C. for 10-15 minutes).
As mentioned herein before, decoction mashing can advantageously be used to produce mash extracts from malt and starch-containing adjuncts. Hence, it would be beneficial if a continuous decoction mashing method could be made available that can suitably be used for producing a mash extract from malt and one or more starch-containing adjuncts.